Several proposals have been made on valve devices of the type capable of automatically stopping the liquid supply when the surface of pressurized liquid supplied to a container has reached a predetermined level. However, all of these proposed valve devices are complicated and not compact in structure and also the valve operation is rather sluggish and not certain. The conventional stop valves of this type, therefore, are weak in liquid blocking force and slow in the stopping operation, thus requiring much time until a perfect stoppge of liquid supply is attained. For instance, in the known automatic constant-level water supply system for water tanks, an operating lever for opening and closing the water discharge valve device at the tank bottom is provided on the tank wall and a ball tap for connecting or disconnecting the water supply pipe and a flooding pipe is provided on a side of the water discharge valve unit in the tank, so that this system has, besides the abovementioned operational defects, the problem that it is costly to manufacture and vulnerable to trouble owing to the complicated structure of the ball tap. Also, the use of such ball tap necessitates enlargement of the tank size.